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High-value alcohols and higher-oxidation-state compounds by catalytic Z-selective cross-metathesis

Olefin metathesis provides access to molecules that are indispensible to research in chemistry, biology and medicine. Despite the recent discovery of catalysts that promote Z-selective reactions, an important persisting problem has been the lack of transformations that directly generate acyclic Z allylic alcohols. This shortcoming extends to products that contain a hindered neighboring substituent and/or reactive and ubiquitous functional units such as a phenol, an aldehyde, or a carboxylic acid. In a recent report in the journal Nature (2015, 517, 181–186; doi:10.1038/nature14061) Professor Amir Hoveyda and graduate studens Ming Joo Koh, Kashif Khan, Miao Yu and Malte Mikus together with postdoctoral associate Dr. Sebastian Torker report on the design of a Ru complex that is uniquely effective in generating the aforementioned compounds by cross-metathesis processes. The complex is prepared from a commercially available Ru-based precursor and an easily generated air-stable zinc catechothiolate. Transformations typically proceed with 5.0 mol % of the complex and an inexpensive reaction partner in 4–8 hours under ambient conditions; products are obtained in up to 80% yield and 98:2 Z:E selectivity. Utility is demonstrated by application to synthesis of naturally occurring anti-tumor agent neopeltolide and a single-step stereoselective gram-scale conversion of a renewable feedstock (oleic acid) to an anti-fungal agent.

To view the publication see: http://www.nature.com/nature/journal/v517/n7533/full/nature14061.html


Bis-BN Cyclohexane: A Remarkably Kinetically Stable Chemical Hydrogen Storage Material

A critical component for the successful development of fuel cell applications is hydrogen storage. For back-up power applications, where long storage periods under extreme temperatures are expected, thermal stability of the storage material is particularly important. In their recent communication from the Journal of the American Chemical Society, Professor Shih-Yuan Liu and his coworkers at Boston College, in collaboration with team members at the Pacific Northwest National Laboratory (PNNL) and The University of Alabama describe the synthesis and characterization of an unusually kinetically stable chemical hydrogen storage material with a H2 storage capacity of 4.7 wt%. The compound, which is the first reported parental BN isostere of cyclohexane featuring two BN units, is thermally stable up to 150 °C both in solution and as a neat material. Yet, it can be activated to rapidly desorb H2 at room temperature in the presence of a catalyst without releasing other detectable volatile contaminants. Furthermore, the team also intercepted and structurally characterized two cage compounds with the unique S4 symmetry that were also formed during the H2 desorption process. The remarkable kinetic stability exhibited by this new compound (a.k.a Bis-BN cyclohexane) and the ability to readily release its stored H2 content under mild conditions would make it a potentially suitable candidate for stationary fuel cell applications. Overall, this work provides a new approach for the development of thermally stable chemical hydrogen storage materials and is establishing the potential for Bis-BN cyclohexane to serve as a monomer for new polymeric architectures.

To view the publication see: http://pubs.acs.org/doi/abs/10.1021/ja511766p


Two BN Isosteres of Anthracene: Synthesis and Characterization

Acenes, or linear benzofused polyaromatics, are a distinct class of polycyclic aromatic hydrocarbons (PAHs) with favorable properties for use in organic materials applications. By replacing an aromatic carbon-carbon bond with an isosteric boron-nitrogen bond (BN/CC isosterism), it is possible to create new chemical space and impart new functions that are distinct from the ones exhibited by corresponding classic carbonaceous PAHs. In their recent article from the Journal of the American Chemical Society, Professor Shih-Yuan Liu and co-workers, in collaboration with the group of Professor Anna Chrostowska (Universitéde Pau et des Pays de l’Adour, France) synthesized two BN isosteres of anthracene, a simple acene containing three fused benzene rings. The authors demonstrate that BN/CC isosterism lowers the energy of the frontier orbitals of the anthracene framework, significantly increasing its stability. This property is important when considering materials for organic optoelectronic materials, since higher acenes often deleteriously react when used in organic electronics under ambient conditions, causing a decrease in performance. Interestingly, the researchers found that despite the changes in frontier orbital energies, the optical properties of the BN anthracenes remain similar to that of the all- carbon anthracene. The work demonstrates that BN/CC isosterism can be used as a molecular design strategy to improve the stability of acene-type structures while maintaining the photophysical characteristics of this important family of compounds.

To view the publication see: http://dx.doi.org/10.1021/ja508813v


Catalytic Enantioselective Multicomponent Assembly

Efficient catalytic reactions that generate C–C bonds enantioselectively and those that produce trisubstituted alkenes diastereoselectively are central to research in modern chemistry. Transformations that accomplish these tasks simultaneously in a single operation are scarce and highly prized; this is particularly the case if the catalysts, substrates and reagents are easily accessed at low cost and reactions are simple to set and conditions are conditions are mild. In an Article in this week’s issue of Nature, Professor Amir Hoveyda and graduate students Fanke Meng and Kevin McGrath report a facile multicomponent catalytic process that begins with a chemo-, site- and diastereoselective copper–boron addition to a monosubstituted allene. The resulting boron-substituted organocopper intermediates then participate in a chemo-, site- and enantioselective allylic substitution. Products, which contain a stereogenic carbon center, a mono-substituted alkene and an easily modifiable Z-trisubstituted alkenylboron group, are obtained in up to 89% yield, with >98% branch- and stereoselectivity and >99:1 enantiomeric ratio. The copper-based catalyst is derived from a robust heterocyclic salt that can be prepared in multi-gram quantities from inexpensive starting materials and without costly purification procedures. Utility of the approach is showcased through exceptionally concise enantioselective synthesis of gram quantities of natural products rottnestol (member of an antibiotic family) and herboxidiene (anti-tumor). In the case of rottnestol, the overall yield is seven times better than the best formerly reported approach, and herboxidiene is accessed in nearly twice the total yield of the most efficient previous route.

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A discussion on the significance of the paper's findings by Professor Phil S. Baran and graduate student Matthew T. Villaume, Scripps Research Institute, appears in Nature: News and Views; To read the article, go to:

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Molecular Encapsulation beyond the Aperture Size Limit through Dissociative Linker Exchange in Metal–Organic Framework Crystals

Incorporating functional guest molecules into the cavities of crystalline porous materials makes it possible to engineer these materials for drug delivery, sensing, electrical conductivity, luminescence, and energy conversion. Recently, attention has been drawn to a molecular-type class of crystalline porous materials, metal-organic frameworks (MOFs) due to their chemically tunable pore surfaces, their comparatively mild syntheses, and their unique properties such as framework flexibility, postsynthetic modification, and exchangeable ligands. Despite these advances, approaches for incorporating large and more diverse guests into MOFs are still limited. In a recent report in the Journal of the American Chemical Society, a team led by Professors Frank Tsung and Jeff Byers introduce a new concept for incorporating larger and more diverse guest molecules into MOFs. In this approach, they take advantage of ligand exchange reactions to “open” part of the framework of the pre-synthesized MOF crystals. Expanded apertures created by the ligand exchange process allow large guest molecules to diffuse into the MOF pore. After guest incorporation, association of the ligand closes the large aperture, trapping the guest molecule in the MOF pore. This new approach to guest incorporation is expected to be general because framework ligand exchange has been carried out under various conditions and exists in a large number of MOFs with diverse secondary building units. An additional practical advantage of decoupling encapsulation and MOF synthesis is that MOF production can be scaled-up independently of guest loading, which is especially relevant since several MOFs, such as ZIF-8, Fe-BTC,
HKUST-1, and MIL-53(Al), have been become commercially available.

To view the publication, see: http://pubs.acs.org/doi/abs/10.1021/ja5054779


Surfactant-Directed Atomic to Mesoscale Alignment: Metal Nanocrystals Encased Individually in Single-Crystalline Porous Nanostructures

IIn colloidal syntheses, it is a challenge to control the lattice alignment at the interface between materials with crystal lattices in disparate scales, such as those between metal lattices and microporous or mesoporous materials, where the structural parameters can differ by orders of magnitude; however, the alignment is highly desired for many applications. In a recent report in the Journal of the American Chemical Society, Professor Frank Tsung and coworkers address this challenge by a new concept inspired by studies on self-assembled monolayers (SAM). By using self-assembled surfactant layers, they have aligned the lattices at the interface in a proof-of-concept structure, a core-shell nanocomposite with a well-defined metal nanocrystal core and a metal-organic-framework (MOF) shell. This system is representative because the metal and MOF lattices differ in structural dimensions by orders of magnitude. This is the first example of utilizing self-assembled molecules in colloidal syntheses to align crystalline materials from the atomic scale to the mesoscale. The alignment is essential for follow-up studies on the diffusion and orientation of molecules in such structures which will be investigated via spectroscopic analysis and catalysis experiments.

To view the publication, see: http://dx.doi.org/10.1021/ja5048522


Organoboron Transformations via Boron-Stabilized Carbanions

Organoboron compounds can be readily converted to a range of valuable products and are thus widely used in chemical synthesis. A pressing issue in modern organic chemistry, therefore, relates to the development of protocols that furnish access to these entities, particularly if readily available and previously unutilized starting materials can be employed. In a recent report in the Journal of the American Chemical Society, Professor James Morken and co-workers outline precisely one versatile instance of such a transformation. The newly developed strategy involves the alkoxide-promoted reaction of 1,1-bis(pinacolboronate) esters with alkyl halides by a pathway that results in the deletion of one of the two geminal boryl units, followed by C–C bond formation of the resulting boron-stabilized carbanion with an electrophile. The process can be accomplished easily on preparative scale (for example, >5 grams), is broadly applicable, and can be accomplished intramolecularly to furnish cyclic products. Mechanistic investigations through the use of 13C- and 10B-labeled compounds support the contention that transformations occur via boron-stabilized carbanions, likely stabilized by π-bonding between the carbanion and the remaining boron (B=C bonding). The research carried out in the Morken laboratories was funded by a grant from the National Institutes of Health.

To view the publication, see: http://pubs.acs.org/doi/abs/10.1021/ja505455z


Binding Affinity of p53: A Tumor Suppressor Peptide

The transcription factor p53 commences arrest of the natural cell cycle in response to DNA damage. The p53 peptide, like other short peptide segments of a protein, is found to be in disordered states in solution and therefore is not able to maintain proper interactions for binding. Stabilization of the 16-residue helical domain in p53 was previously accomplished by introducing an all-hydrocarbon tether of different lengths and stereochemistry. This stapled version of the p53 peptide was shown to slow the growth of cancer cells in vivo by activating the p53-mediated apoptotic paths.

Through modeling and binding affinity calculations, Professor Udayan Mohanty recently characterizes the effects of varying the peptide sequence and different parameters of the hydrocarbon bridge. The WaterMap method identified over one hundred hydration sites in the protein binding pocket and calculated the relative value of free energy released by displacing the hydration sites. The potentials of mean force obtained by the weighted histogram analysis method were calculated along a reaction coordinate for p53 binding to MDM2. He and his students found their binding affinity methods to be in agreement with experimental data and they correctly identify the order of the peptide variants from lowest to highest binding affinity. Their efforts are expected to help to further the development of a new generation of p53/MDM2 inhibitors that can reactivate the function of p53 as a tumor suppressor gene.


A Significant Advance in Chemical Synthesis: Sequential Catalytic and Enantioselective Formation of C–B and C–C Bonds

Terminal alkenes are among the most desirable starting materials in organic synthesis: they are manufactured on enormous scale and can be used in countless transformations.  While their latent reactivity readily lends itself to hydrocarbon chain extension, alkenes have the attractive feature of being stable to many acids, bases, oxidants and reductants.  Despite such attractive attributes, catalytic enantioselective transformations that utilize mono-substituted alkenes and proceed in high enantioselectivity (e.g., 95:5 enantiomer ratio or better) are scarce.  In a recent publication in Nature, Professor James Morken along with senior graduate students Scott Mlynarski and Chris Schuster, outline a remarkable single-flask catalytic enantioselective process that transforms terminal alkenes into a range of useful and valuable products in high enantiomeric purity.  The BC approach involves an all-catalytic sequence that includes a diboron addition and is followed by a regioselective cross-coupling to generate a C–C bond.  A large assortment of otherwise difficult-to-access products can thus be generated in >95:5 enantiomer ratio by reactions that are promoted by no more than 1-2 mol % of commercially available catalysts and reagents.

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Surface Modification Increases Photovoltage Generation by "Rust" By Over 100%

Artificial photosynthesis seeks to harvest solar energy and directly convert it into chemical energy for easy storage and redistribution. In developing technologies to meet this goal, scientists face an important challenge that photoelectrodes often fail to generate enough voltage to power desired chemical reactions, such as the splitting of water into molecular oxygen and hydrogen. The challenge is probably best manifested by a prototypical material, hematite (α-Fe2O3), which can only produce 0.3 V power while its bandgap is >2 V. Professor Dunwei Wang and his team show that the issue can be mitigated by a simple surface modification. Using a recently identified oxygen evolution catalyst, the Wang group succeeded in measuring a photovoltage of 0.61 V (or an increase of 100%) on hematite. As a result, a record-low turn-on voltage of 0.62 V (vs. reversible hydrogen electrode) was observed. When combined with silicon nanowire dual absorbers, the turn-on potential was further reduced to 0.32 V (corresponding to a photovoltage of 0.91 V). These data, published in Angewendte Chemie International Edition, represent a new benchmark for the hematite-based solar water splitting reactions. Most significantly, the team considers the cause of the cathodic shift a result of improved thermodynamics, a dramatic deviation from previous views that such a shift would be a result of kinetic changes. The conclusion is supported by their photovoltage measurements under open circuit conditions where the influence of kinetic factors is nonexistent. The result renews the hope of using hematite, an earth abundant, inexpensive, and stable material, for practical artificial photosynthesis. This research was supported financially by the National Science Foundation and the Alfred P. Sloan Foundation.

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Enantioselective silyl protection of alcohols promoted by a combination of chiral and achiral Lewis basic catalysts

In Nature Chemistry, Professors Amir Hoveyda and Marc Snapper and their coworkers describe and demonstrate a counterintuitive strategy for dramatically accelerating the catalytic enantioselective monosilylations of diols and polyols to furnish valuable alcohol-containing molecules in high enantiomeric purity. In the past, such transformations typically required high catalyst loadings (20–30 mol%) and long reaction times (2–5 days). The team’s new strategy involves the addition of an achiral co-catalyst that is structurally similar to the chiral catalyst, which effectively solves the problem. The key discovery is that a combination of seemingly competitive organic bases can function in concert, such that one serves as an achiral nucleophilic promoter while the other performs as a chiral Brønsted base. On the addition of 7.5–20 mol% of a commercially available N-heterocycle (5-ethylthiotetrazole), reactions typically proceed within one hour and deliver the desired products in high yields and high enantiomeric ratios. In some instances, there is no reaction at all in the absence of the achiral base, yet addition of the achiral co-catalyst gives rise to facile formation of products in high enantiomeric purity. The principles outlined in this paper serve as a conceptual framework for the development of new processes that demand separate and independently operational Lewis basic co-catalysts whose functions can easily overlap and the simultaneous use of which might initially appear to be detrimental towards achieving high enantioselectivity. This research was supported financially by the National Institutes of Health.

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A grossly warped nanographene and the consequences of multiple odd-membered ring defects

Graphite, the most stable form of elemental carbon, consists of pure carbon sheets stacked upon one another like reams of paper. Individual sheets, known as graphene, prefer planar geometries as a consequence of the hexagonal honeycomb-like arrangements of trigonal carbon atoms that comprise their two-dimensional networks. Defects in the form of non-hexagonal rings in such networks cause distortions away from planarity. In Nature Chemistry, Professor Lawrence T. Scott reported an extreme example of this phenomenon. A 26-ring C80H30 nanographene that incorporates five seven-membered rings and one five-membered ring embedded in a hexagonal lattice was synthesized by stepwise chemical methods, isolated, purified and fully characterized spectroscopically. Its grossly warped structure was revealed by single-crystal X-ray crystallography. An independent synthetic route to a freely soluble derivative of this new type of "nanocarbon" was also reported. Experimental data reveal how the properties of such a large graphene subunit are affected by multiple odd-membered-ring defects. This research was carried out in collaboration with Dr. Kenichiro Itami, Professor of Chemistry and Director of the Institute of Transformative Bio-Molecules at the University of Nagoya, Japan. The other authors of this paper include Dr. Yasutomo Segawa, an assistant professor at the University of Nagoya, Dr. Qianyan Zhang, a post-doctoral research fellow at Boston College, and Katsuaki Kawasumi, a Ph.D. student from Nagoya, who worked for three months during the course of this project as a visiting Ph.D. student at Boston College. The research was supported financially by the National Science Foundation and the Japan Society for the Promotion of Science.

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Readily Accessible and Easily Modifiable Ru-Based Catalysts for Efficient and Z-Selective Ring-Opening Metathesis Polymerization and Ring-Opening/Cross-Metathesis

In a recent publication in the Journal of the American Chemical Society, Professor Amir Hoveyda and his coworkers reported rationally designed Ru-based catalysts for efficient Z-selective olefin metathesis. The new complexes contain a dithiolate ligand and can be accessed in a single step from commercially available precursors in 68–82% yield. High efficiency and exceptional Z-selectivity (93:7 to >98:2 Z:E) were achieved in ring-opening metathesis polymerization (ROMP) and ring-opening/cross-metathesis (ROCM) processes; the transformations typically proceed at 22 °C and are operationally simple to perform. Complete conversion was observed with catalyst loadings as low as 0.002 mol %, and turnover numbers of up to 43 000 were achieved without rigorous substrate purification or deoxygenation protocols. X-ray data and density functional theory computations provide support for key design features and shed light on mechanistic attributes. This research was supported financially by the National Science Foundation, a LaMattina Graduate Fellowship to Mr. R. Kashif M. Khan, and a Swiss NSF Postdoctoral Fellowship to Dr. Sebastian Torker.

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1,3,5-Triazine as a Modular Scaffold for Covalent Inhibitors with Streamlined Target Identification

Cysteine residues play key functional roles in regulating protein activity by acting as catalytic nucleophiles, sites of metal-binding, and redox-active centers. Small molecules that covalently modify these functional cysteine residues provide pharmacological tools to perturb protein activity within biological systems. In a recent publication in the Journal of the American Chemical Society, Professor Eranthie Weerapana and her coworkers reported the utility of trifunctionalized 1,3,5-triazine as an ideal modular scaffold to generate covalent inhibitors for cysteine-mediated protein activities. Triazines were derivatized with a thiol-reactive electrophile for covalent modification of target proteins, an alkyne as a click-chemistry handle for target identification, and a binding group to direct the compounds toward distinct subsets of the proteome. A library of trifunctionalized triazines was generated, and the cellular protein targets for these compounds were evaluated. These cellular screens identified two compounds, RB-2-cb and RB-11-ca, as cell permeable and highly selective covalent modifiers for Cys239 of beta-tubulin (TUBB) and Cys53 of protein disulfide isomerase (PDI), respectively. These compounds demonstrate in vitro and cellular potencies that are comparable to currently available modulators of tubulin polymerization and PDI activity. These findings demonstrate the versatility of the triazine core as a modular scaffold to generate potent and selective covalent modifiers of diverse protein families for future chemical genetics applications. Future studies will apply these bioactive small molecules to further interrogate the role of TUBB and PDI in a variety of pathological systems and expand the triazine library to target other reactive amino acids in the proteome. Postdoctoral fellow Ranjan Banerjee, graduate student Nicholas Pace, and undergraduate student Douglas Brown contributed to this study.

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Simple Organic Molecules as Catalysts for Enantioselective Synthesis of Amines and Alcohols

Discovery of easily accessible catalysts that generate high value organic compounds by sustainable enantioselective transformations is central to advances in the life sciences. In the February 14 issue of Nature, in a paper by various members of the Hoveyda research group, graduate students Dan Silverio and Erika Vieira, postdoctoral fellows Drs. Sebastian Torker and Tanya Pilyugina, and senior research associate Dr. Fredrik Haeffner introduce a set of small organic molecules that catalyze reactions of unsaturated organoboron reagents with imines and carbonyls; products are amines and alcohols of high enantiomeric purity, intermediates used to synthesize many biologically active molecules. A distinguishing feature of the catalyst class is a proton embedded within their structure. The resulting electronic activation and structural organization play a key role in every stage of the carbon-carbon bond forming processes; this includes achieving high rates of catalyst regeneration and product release, typically obtained through rapid ligand exchange with metal-containing systems. The catalyst is derived from the abundant amino acid valine and can be prepared in large quantities in four steps with cheap chemicals. Reactions are scalable, do not demand stringent conditions, can be performed with as little as 0.25 mole % catalyst in less than six hours at room temperature, and furnish products typically in >85% yield and ≥97:3 enantiomeric ratio. The efficiency, selectivity and operational simplicity of the transformations and the range of compatible boron-based reagents render this advance vital to future progress in chemistry, biology and medicine. The research is part of a longstanding collaboration (since 1995) between the research groups of Professor Amir Hoveyda and Professor Marc Snapper in our department.

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Selectivity Control of Heterogeneous Catalysis: Designing Selective Reaction Pores on Metal Surfaces

Metal nanoparticle heterogeneous catalysts are used in most of the industrial chemical processes because they are easy to handle, separate, recover, and reuse. Unfortunately, it is much more challenging to control the selectivity and activity of heterogeneous catalysts compared to homogeneous catalysts. To tackle this challenge, Assistant Professor Frank Tsung and his students have developed a new concept to control heterogeneous catalysis by coating the catalyst with a layer of nanoporous shell. The pores in this skin-like shell modulate the reactions, just like the active packets in enzymes regulate biochemical reactions.

A heterogeneous catalytic process typically involves three important steps: the adsorption of reactant molecules on the active metal surface, reaction between reactant molecules on the metal surface, and, finally, desorption of products. In Prof. Tsung’s design, a nanoporous material, a metal organic framework (MOF), is coated on the surface of metal nanoparticles. Because the size of the MOF pores is comparable to that of the reactant molecules, the MOF could concurrently manipulate the molecular size-selectivity, adsorption geometries and sorption energies of reactant molecules on the catalyst surface. The Tsung group has applied this catalyst to the gas-phase hydrogenation of ethylene, cyclohexene, and cyclooctene. The size of ethylene is smaller than the pore size of the MOF; the size of cyclohexene is similar to the pore size, and the size of cyclooctene is much bigger than the pore size. The MOF shell provides excellent molecular-size selectivity. The results show high activity in ethylene and cyclohexene hydrogenations but not in cyclooctene hydrogenation. Different activities for cyclohexene hydrogenation were obtained for the catalysts with and without the MOF shell. The difference could be due to the conformation of the molecules on the metal surface during the hydrogenation. The cyclohexene molecules could only interact with the active metal surface by the successive variation in their conformations. None of these enzyme-like behaviors have ever been previously observed in heterogeneous catalysis. The work has been published in J. Am. Chem. Soc. 2012, 134, 14345-14348.

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Magnesium Fluctuations Modulate RNA Dynamics in the Sam-I Riboswitch

Experiments demonstrate that Mg2+ is crucial for structure and function of RNA systems, yet the detailed molecular mechanism of Mg2+ action on RNA is not well understood. In collaboration with the research groups of Jose Onuchic (UCSD/Rice) and Karissa Sanbonmatsu (Los Alamos), Professor Udayan Mohanty has examined the interactions between RNA and its ion atmosphere at atomic resolution in the SAM-I aptamer. Specifically, they have carried out ten 2 µs explicit solvent molecular dynamics simulations and determined the excess number of ions around RNA as a function of varying ion concentrations. This quantity, which is experimentally measurable, is called the preferential interaction coefficient. Surprisingly, the investigators found that between 80% and 85% of the excess Mg2+ ions that contribute to the preferential interaction coefficient reside in the outer-sphere layer. The electronegative RNA atoms and the hydration layers both play a role in situating the excess Mg2+ ions. Furthermore, the outer-sphere Mg2+ ions exhibit 100-fold slower kinetics than the Mg2+ ions in the diffuse layer. As the outer-sphere accounts for most of Mg2+, it changes the paradigm of the characteristics of RNA-Mg2+ interactions. The work has been published in J. Am. Chem. Soc. 2012, 134, 12043–12053.

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Transitions from Functionalization to Fragmentation Reactions of Secondary Organic Aerosol (SOA) Generated from OH Oxidation of Alkaline Precursors

The importance of atmospheric aerosols as climate forcing agents has become evident. The quantitative parameters required for reliable modeling aerosol climate effects, however, are not yet available. This is most notably the case for organic aerosol. To address this problem, Professor Paul Davidovits and his collaborators have been systematically studying the formation of organic aerosol via atmospheric oxidation of organic vapors due to anthropogenic and biogenic emissions. They measure cloud formation effectiveness and optical properties of organic aerosols of known shape, size, and composition. They also determine how these properties are affected by transformations in the atmosphere via oxidative reactions and coatings formed by deposition.

The experiments described in their publication in Environmental Science & Technology 2012, 46, 5430 were designed to study aerosol formation from gas phase alkanes emitted into the atmosphere as a result of oil spills such as the Deepwater Horizon accident. The experiments provided data that determine yields and chemical composition of the aerosol produced from atmospheric oxidation of the relevant gas phase species.

These studies were conducted in Professor Davidovits’s laboratories at Boston College in collaboration with colleagues from Aerodyne Research Inc., MIT, and Pennsylvania State University. The wide range of instruments and expertise required to perform complex atmospherically relevant studies make such collaborations essential. Three Boston College undergraduate students, David R. Croasdale, Justin P. Wright, and Alexander T. Martin, were important participants in these experiments and are co-authors of the publication.

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Efficient Syntheses of 6,5'-(S)- and 6,5'-(R)- Cyclouridine

Professor Larry W. McLaughlin and graduate student Christopher S. Theile have successfully synthesized both the 5’-R and the 5’-S diastereomers of 6,5’-cyclouridine. Cyclonucleosides feature an extra bond between the backbone/sugar portion of the nucleoside and the base. These compounds are products formed in vivo from oxidative damage induced from radiation exposure or reactive oxygen species. The work has been published in Chem. Commun. 2012, 48, 5587.

Two diastereomers can be formed in the cyclization reaction. The 5’-S isomer has its 5’ hydroxyl group positioned over the sugar portion of the nucleoside, whereas the 5’-R compound has its hydroxyl group pointed away. These compounds have been very difficult to synthesize, especially the 5’-R isomer. The McLaughlin lab synthesis features an efficient oxidation step that generates both isomers at once, but favors the 5’-R compound over the 5’-S in a nearly 2:1 ratio. X-ray crystal structures of the compounds were also obtained, which show that the nucleobase is “pulled” back from its normal position, preventing it from forming hydrogen bonds in its native fashion.

Now that these compounds have finally been synthesized in the laboratory in an efficient manner, other scientists can begin using them to probe enzymes and biological systems, in order to understand better the damage that these cyclonucleosides can cause.

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Dual Absorber Hematite/Si Nanowire System for Photoelectrochemical Water Splitting at Low Applied Potentials

Using the sun to split water into oxygen and hydrogen, which could be used for fuel, remains elusive. However, a new study by Associate Professor Dunwei Wang and co-workers brings this goal tantalizingly closer by crafting silicon nanowires coated with hematite (alpha-Fe2O3) that work synergistically to collect photons from different areas of the solar spectrum for conversion into electricity.

Such sun-powered water splitting, also known as photoelectrochemical (PEC) water splitting, would be an efficient way to collect solar energy and use it in the form of hydrogen fuel. The choice of the photoelectrode, the component that collects solar energy and converts it into electricity, is especially important because it determines the water-splitting device’s performance over time. The ideal photoelectrode would absorb light broadly, be inexpensive, and resist photocorrosion. Hematite fits some of these characteristics, but alone, it doesn’t perform well enough for consideration.

To improve hematite’s performance, Professor Wang’s research team grew crystals of this iron oxide on silicon nanowires, a material that absorbs light that is transparent to hematite. Together, the two materials absorb light from a much larger portion of the solar spectrum and efficiently convert that energy into electricity. The fact that the photoelectrode’s active materials are “primarily composed of three of the four most abundant elements in Earth’s crust (O, Si, and Fe) offers promise that renewable energy harvesting by PEC water splitting remains an achievable goal”, Professor Wang concludes.

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Silicon Nanowires as Photoelectrodes for Carbon Dioxide Fixation

Natural photosynthesis harvests solar energy and stores it in chemical bonds. The synthesis typically proceeds in two steps, a light reaction that splits water to produce dioxygen molecules, followed by dark reactions that fix carbon dioxide and yield organic compounds with high yield and specificity. In an effort to develop technologies that can meet our ever-increasing energy demand in a renewable fashion, scientists and engineers have researched heavily to understand and mimic these reactions. To date, the focus has been mainly on how to harvest solar energy efficiently; however, the other aspect of using the energy converted from photons to produce synthetically valuable molecules is a far more challenging problem. An important reason for the challenge is the rich oxidation states of carbon, from -4 (e.g., in CH4) to +4 (in CO2), and the fact that the energy difference between reactions that would produce carbon with various oxidation states is typically too small to allow for high selectivity.

Using a photoelectrode made of Si nanowires and employing ketones as electron-receiving substrates, Assistant Professors Wang and Tan teamed up to demonstrate a proof-of-concept that may provide a solution to the challenge. In close similarity to natural photosynthesis, their strategy seeks to carry out the reactions in two steps. First, light strikes Si nanowires to excite electrons, which transfer to ketones such as benzophenone to produce radical anions. Then, the radical anions react with carbon dioxide in a highly specific manner (see Figure above). After a second electron transfer from the Si nanowire photoelectrode, the carbon fixation process is complete to produce carboxylic acids. The advantages of this strategy include high energy conversion efficiency, durability of the photoelectrode, a cheap carbon source, as it is from carbon dioxide, high chemical yields, and high specificities of the reactions, the last which of is demonstrated in the synthesis of precursors for ibuprofen and naproxen.

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The First Book about Geodesic Polyarenes, Carbon Nanorings, and Aromatic Hydrocarbon Belts

The book Fragments of Fullerenes and Carbon Nanotubes: Designed Synthesis, Unusual Reactions, and Coordination Chemistry grew out of a very successful symposium on the same topic that was held in conjunction with the 2008 national meeting of the American Chemical Society in Philadelphia. Shortly after that meeting, an editor from John Wiley & Sons approached Professor Lawrence T. Scott and Professor Marina Petrukina from the University at Albany, who had coorganized the symposium, about publishing a book on this burgeoning branch of chemistry. The two agreed to accept the challenge, and each contributed a chapter to the book from their own research. The other chapters in this edited monograph were written by prominent experts in the field from all over the world, many of whom had spoken in the symposium. The chapter written by Professor Scott on “Hemispherical Geodesic Polyarenes: Attractive Templates for the Chemical Synthesis of Uniform Diameter Armchair Nanotubes” reports previously unpublished results from the Ph.D. thesis of former BC graduate student Anthony Belanger, as well as contributions from the undergraduate research of former BC chemistry major and Scholar of the College Katherine Mirica and early groundwork by former postdoctoral fellow Dr. James Mack. The front cover of the book is adapted from “La Tricoteuse,” a painting by William Adolphe Bouguereau (1825-1905), digitally modifed to show the girl knitting a scarf in the likeness of a carbon nanotube.

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The First Chemical Synthesis of a Short, Rigid, Structurally Pure Carbon Nanotube

The discovery of carbon nanotubes in 1991 almost instantly inspired dreams among scientists and engineers about how these light weight, super strong, heat resistant, ultra thin nanowires could someday be used to miniaturize electronic devices down to the nanometer scale, well beyond the limits of what can ever be achieved by lithography on silicon chips. Unfortunately, not all carbon nanotubes (CNTs) are alike. Some are semiconductors, whereas others are highly conductive, like metals, with current carrying capacities up to 1000 times greater than that of copper wire. Despite two decades of extensive experimentation worldwide, however, all known preparation methods still yield mixtures of different types of CNTs that are virtually impossible to separate into their individual components in useful amounts. Stimulated by the challenge to address this long-standing problem, Professor Lawrence Scott and his students began exploring a revolutionary new production strategy involving the controlled elongation of small hydrocarbon templates, such as hemispherical nanotube end-caps, prepared by bottom-up chemical synthesis; the diameter and rim structure encoded in the template would dictate the diameter and chirality of the resulting CNT. Toward that objective, the Scott laboratory has succeeded in synthesizing a short CNT by chemical methods in just three steps from corannulene, a readily available starting material. The new C50H10 geodesic polyarene has been isolated, purified, crystallized, and fully characterized by NMR spectroscopy, UV-vis absorption spectroscopy, high resolution mass spectrometry, and X-ray crystallography. With this advance, a major obstacle to the rational synthesis of structurally uniform CNTs has been overcome. Only now that a small hydrocarbon template such as this is finally available can methods for elongating it into uniform CNTs be tested or invented. Work on this final phase is currently in progress. Financial support was provided by the National Science Foundation and the Department of Energy.

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A Practical and Catalytic Method that Provides Access to Many Biologically Active Molecules

A large number of biologically active macrocycles contain a C–C double bond through which various other derivatives are prepared; the stereochemical identity of the alkene or the resulting moieties can be critical to the beneficial properties of such molecules. Catalytic ring-closing metathesis (RCM) is a widely employed method for the synthesis of large unsaturated rings; however, cyclizations often proceed without control of alkene stereochemistry. Such shortcoming is particularly costly with complex molecules when cyclization is performed after a long sequence of transformations. In the most recent issue of Nature (2011, 479, 88–93; doi:10.1038/nature10563), Professor Amir Hoveyda, graduate student Miao Yu and postdoctoral fellow Chenbo Wang disclose a practical and general approach for efficient and highly stereoselective synthesis of macrocyclic alkenes by catalytic RCM; transformations deliver up to 97% Z selectivity due to control induced by a tungsten-based alkylidene. The exceptional utility of the method is demonstrated by stereoselective preparation of highly potent anti-cancer agents epothilone C and nakadomarin A, previously reported syntheses of which have been marred by late-stage non-selective RCM. The team reports that the tungsten alkylidene can be manipulated in air, promoting reactions carried out in a fume hood to deliver products in useful yields and high Z selectivity. As a result of efficient RCM and re-incorporation of side products into the catalytic cycle with minimal alkene isomerization, desired cyclizations proceed in preference to alternative pathways even under relatively high concentration (0.1 molar). The research, conceived and largely performed in the Boston College laboratories by Wang and Yu, is in collaboration with Professors Richard Schrock (MIT) as well as Professor Darren Dixon (Oxford) and two members of his team. Financial support was provided by the National Institutes of Health and the UK ESPRC.

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Novel Imaging Agents of Cell Death

Essentially all anticancer drugs work by directly or indirectly inducing apoptosis of cancer cells, the programmed cell death pathway. Therefore, noninvasive imaging of apoptotic cells should enable prompt evaluation of the efficacy of cancer therapeutics. In principle, apoptotic cell death can be reliably detected by targeting the lipid molecule phosphatidylserine (PS), which in healthy cells is completely confined to the cytosolic side of the plasma membrane. In apoptosis, the distribution asymmetry is lost, resulting in PS exposure on the surfaces of dying cells. Natural PS-binding proteins are less than ideal as imaging agents of apoptosis for multiple reasons, including their large size, low stability, difficulty of labeling, and their poor tissue penetration. Small molecule ligands that specifically target PS may circumvent these problems and thereby serve as powerful imaging tools for cancer research and treatment. In their recent paper in the Journal of the American Chemical Society, Professor Jianmin Gao and coworkers report a new class of PS-targeting molecules that enables facile imaging of apoptotic cells. These PS-imaging agents are developed by using a biomimetic approach: the PS-binding epitope of the milk protein lactadherin are grafted onto a cyclic peptide scaffold; rational optimization using unnatural amino acids results in the cyclic lactadherin mimics (cLacs) that preferentially associate with PS-presenting membranes. The cLac design also benefits from the intrinsic membrane impermeability of peptides, which allows cLacs to target surface-exposed PS. A fluorophore labeled cLac effectively detects apoptotic cells under a fluorescence microscope. Given the small size and ease of synthesis and labeling, cLacs hold great promise for noninvasive imaging of cell death in living animals and ultimately in patients.

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Kantrowitz Lab uses X-ray Crystallography to Provide a Molecular Level Description of Each of the Steps in the Catalytic Cycle of a Critical Metabolic Enzyme

Graduate students Katharine Harris and Gregory Cockrell, along with undergraduate David Puleo, working with Professor Evan Kantrowitz have used protein crystallography and theoretical studies to provide a detailed mechanism of the reaction catalyzed by the enzyme aspartate transcarbamoylase. This enzyme catalyzes one of the first reactions in the biosynthesis of the pyrimidine nucleotides, the building blocks of DNA and RNA. This enzyme also helps to control the rate of the entire pathway and has become the target for the development of drugs that can be used to fight cancer and malaria. This work is important because it provides details on the molecular level of the active site of the enzyme and the steps involved in the catalytic reaction, which are critical for drug design. By combining data from three high-resolution X-ray crystal structures of the enzyme in the absence of ligands, in the presence of one of the enzyme’s two substrates, and in the presence of the bisubstrate/transition state analog with theoretical studies involving in silico docking and electrostatic calculations, the Kantrowitz Lab has been able to visualize each step in the catalytic cycle of ATCase, from the ordered binding of the substrates, to the formation and decomposition of the tetrahedral intermediate, to the ordered release of the products from the active site. These results provide a molecular level portrait of how this critical metabolic enzyme functions.

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Tan Lab Extends Its Scaffolding Catalysis Strategy To a Metal-free System for the Enantioselective Desymmetrization of 1,2-Diols

Students working with Professor Kian Tan have recently developed a new non-metal catalyst that uses reversible covalent bonding between catalyst and substrate in order to enhance both reactivity and selectivity. Graduate students Xixi Sun and Amanda Worthy disclosed in Angew. Chem., Int. Ed. that these scaffolding catalysts are highly effective for the enantioselective desymmetrization of 1,2-diols. For most synthetic catalysts, reversible covalent bonding is used to form a reactive intermediate, thereby affording enhanced rate of reaction. In this case reversible covalent bonding is implemented to transiently tether reagents. A key aspect of this mode of catalysis is that a part of the acceleration arises from the entropic advantage gained through intramolecularity. This mode of catalysis is infrequently used in synthetic catalyst designs but holds great promise as a general method for accelerating reactions.

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Snapper Lab Builds Polycyclic Lactones by Powerful Tandem Catalytic Reaction Sequences

David F. Finnegan, working in Professor Marc L. Snapper’s laboratory developed a new ruthenium-catalyzed tandem reaction sequence that generates polycyclic compounds from acyclic precursors in one reaction flask. The process uses a single ruthenium additive to catalyze the two mechanistically distinct transformations; a ring closing metathesis followed by a hetero-Pauson-Khand cycloaddition. The key to developing this tandem process was to learn how to convert in situ the metathesis active ruthenium alkylidene into a hetero-Pauson-Khand cycloaddition catalyst. This was accomplished by treating the metathesis active ruthenium complex with CO to remove the alkylidene functionality, followed by a reductant (methoxide) and more CO to generate the active ruthenium (0) carbonyl complex that catalyzes the desired cycloaddition. This work was published as a “Feature Article” in the Journal of Organic Chemistry. The National Science Foundation provided financial support for these studies.

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A Boron-Based Synthesis of the Natural Product (+)-trans-Dihydrolycoricidine

A synthesis of the natural product trans-dihydrolycoricidine has been accomplished through the aid of new boron-based chemical reactions developed in the laboratory of Professor James Morken. Postdoctoral fellow Dr. Sarah Poe (now at the Warner-Babcock Institute for Green Chemistry) developed a diastereoselective diboration reaction that converts cyclic dienes to single isomer cyclohexendiols (Eq. 1, above). This reaction occurs with excellent levels of stereoselectivity and can accomplish oxidations that have not been possible with singlet oxygen-based methods. Dr. Poe then employed this reaction along with catalytic allylboration reactions that the Morken group has also developed to provide an efficient synthesis of the target molecule. The work has been published in Angewandte Chemie.

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Enantioselective Conjugate Addition of SiMe2Ph Catalyzed by a Chiral N-Heterocyclic Carbene

Research in the laboratory of Professor Amir Hoveyda has resulted in the development of the first catalytic method for forming carbon–silicon bonds that does not require an organometallic complex as the catalyst. This accomplishment is based on a discovery made two years ago in the Hoveyda group, illustrating that achiral N-heterocyclic carbenes (NHCs) – in the absence of a metal salt – can activate a B–B bond to catalyze efficient, but non-enantioselective, boronate conjugate additions to α,β-unsaturated carbonyl compounds. In a recent communication in the Journal of the American Chemical Society, Hoveyda and graduate student Jamie O’Brien disclose a metal-free method for enantioselective conjugate addition of the versatile dimethylphenylsilyl group to a wide variety of α,β-unsaturated carbonyl compounds. Transformations are catalyzed by a chiral N-heterocyclic carbene (NHC) and are performed in an aqueous solution, which makes them operationally simpler to perform than the NHC–Cu-catalyzed variant. The chiral catalyst, also originally developed in the Hoveyda group, is generated from a readily accessible enantiomerically pure imidazolinium salt (prepared in three steps) and a common organic amine base. NHC-catalyzed processes proceed with 5.0-12.5 mol % catalyst loading at room temperature within 1-12 hours, affording the desired products in up to >98:2 enantiomeric ratio and in up to >98% yield. Cyclic enones or lactones and acyclic α,β-unsaturated ketones, esters as well as aldehydes can be used as substrates.

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Catalytic Z-Selective Olefin Cross-Metathesis for Natural Product Synthesis

Alkenes are found in a great number of biologically active molecules and are employed in numerous transformations in organic chemistry. Many olefins exist as E or higher energy Z isomers. Catalytic procedures for the stereoselective formation of alkenes are therefore valuable; nonetheless, methods for synthesizing 1,2-disubstituted Z olefins remain scarce. In an Article published in Nature, Professor Amir Hoveyda and his team report catalytic Z-selective cross-metathesis reactions of terminal enol ethers, which have not been reported previously, and allylic amides, which have been employed until now only in E-selective processes; the resulting disubstituted alkenes are formed in up to >98% Z selectivity and 97% yield. The new transformations are promoted by catalysts that contain the highly abundant and inexpensive molybdenum, and are amenable to gram-scale operations. The Article introduces the use of reduced pressure as a simple and effective strategy for achieving high stereoselectivity. The utility of the new discovery was demonstrated by syntheses of anti-oxidant C18 (plasm)-16:0 (PC), found in electrically active tissues and implicated in Alzheimer’s disease, and the potent immunostimulant KRN7000. The research was performed by postdoctoral fellow Dr. Simon J. Meek, graduate student Robert V. O’Brien, and visiting scholar Josep Llaveria, from Spain, as part of the longstanding collaboration with Professor Richard Schrock at MIT. Funding was provided by the NIH (joint grant to Hoveyda and Schrock) as well as an NSF grant to Hoveyda; O’Brien is a LaMattina graduate fellow, and Llaveria was funded by the Spanish Ministry of Education.

For more details, see also:

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Nature’s Clever Use of Thermal Fluctuations

Recent research published in the Proceedings of the National Academy of Sciences by Professor Udayan Mohanty has advanced our understanding of the clever ways the molecular machinery of life uses fluctuations and dissipation and the large role that thermal fluctuations between structures plays in enzymatic function.

With the advent of single molecule methods, scientists have begun to get a glimpse into how nature has developed its molecular machinery not only to deal with fluctuations, but even to use them to advantage. Single molecule experiments indicate that large and rare thermal fluctuations of the ribosome are essential to rotate the ternary complex into a position so as to facilitate stable contacts with the GTPase activated center (GAC) and the sarcin-ricin loop (SRC) in the large subunit.

In collaboration with Professor Steven Chu at U.C. Berkeley, Professor Mohanty and his students have explored how large, rare thermal fluctuations of the ribosome aid in the functioning of the ribosome. Their theoretical findings establish that configurations, which make significant contribution to the probability of the rare process, have the inherent property that they are localized tightly around the most likely configuration. Small variations in the re-positioning of cognate relative to near-cognate complexes lead to significant rate enhancement.

The protein functional groups, the structural positioning of the RNA, and the inner-sphere coordination of the protein atoms to them, create several unique motifs for the binding of Mg2+ ions in the large subunit of H. marismortuni and E. coli ribosomes. The paper examines over a dozen unique structural motifs of magnesium binding sites to further elucidate the interplay between structure, dynamics and function within the ribosome and to provide insights into how site-bound magnesium ions contribute to its structural stability.

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Zinc(II)-Catalysis of Keteniminium Ion Formation Enables a New Approach to Four-Membered Rings

The Ghosez synthesis of cyclobutanones based on keteniminium salts is a useful variant of the classic ketene–olefin [2+2] cycloaddition reaction. Each process generates four-carbon ring strain and formally accomplishes the vicinal carbofunctionalization of an alkene. Professor Jason Kingsbury and graduate student Jamie O’Brien have now published a full article in The Journal of Organic Chemistry that describes, for the first time, catalysis of keteniminium–alkene cyclocondensation. The new transformations take place with complete regiochemical control at room temperature in the absence of solvent, giving functional cycloadducts with hindered all-carbon-substituted stereocenters. Their studies also include the results of extensive ab initio calculations carried out in collaboration with recent graduate Dr. Adil Zhugralin to help elucidate the reaction mechanism. The authors hope that their work can inspire modern enantioselective entries to substituted cyclobutanones, since prior strategies have relied exclusively on pyrrolidine-based chiral auxiliaries.

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Professor Kian Tan's synthesis of beta-amino-aldehydes

The synthesis of beta-amino-aldehydes has been achieved through enantioselective hydroformylation of PMP-protected allylic amines. The reaction is accomplished by using a scalemic scaffolding ligand that covalently and reversibly binds to the substrate. These ligands behave like chiral auxiliaries because they are covalently attached to the substrate during hydroformylation; however, similar to traditional asymmetric ligands, they can be used in catalytic quantities. The directed hydroformylation of disubstituted olefins occurs under mild conditions (35 °C and 50 psi CO/H2), and Z-olefins afford excellent enantioselectivities (up to 93% ee).

To read the article, go to: http://pubs.acs.org/doi/abs/10.1021/ja107433h

To read a published commentary on the article, go to: http://onlinelibrary.wiley.com/doi/10.1002/anie.201006489/abstract


Professor Lawrence Scott’s article highlighted on the cover of Journal of Materials Chemistry

Carbon nanotubes have been widely touted for their potential to fulfill dreams in materials science and nanotechnology. Despite intense scrutiny by scientists and engineers worldwide for two decades, however, these fascinating carbon-rich materials are still being made today by poorly understood empirical methods that produce inseparable mixtures of tubes, the properties of which vary widely as a function of tube diameter and chirality. This inhomogeneity has seriously impeded realization of numerous applications envisioned in molecular scale electronics that require single-walled nanotubes (SWNTs) with uniform properties. Only those tubes that exhibit metallic properties can serve as electrically conductive nanowires, for example.

Professor Scott and graduate student Eric H. Fort have recently demonstrated that nitroethylene can serve as a highly reactive “masked acetylene” for converting aromatic hydrocarbon bay regions into new, unsubstituted benzene rings by a one-pot Diels-Alder cycloaddition/rearomatization reaction sequence, and this new methodology holds great promise as a strategy for the metal-free growth of single-diameter, single-chirality carbon nanotubes from small hydrocarbon templates (see doi/pdf/10.1021/ja907802g and doi/10.1002/anie.201002859/pdf). In their recently highlighted J. Mater. Chem. article, Fort and Scott report molecular orbital calculations on the Diels–Alder reactivity of aromatic belts and hemispherical end-caps of varying dimensions, and their results offer strong support for the feasibility of this strategy. Bay regions on the rim of a [10,10]nanotube end-cap are predicted to exhibit Diels–Alder reactivity comparable to that of bay regions in planar polyarenes that have already been successfully transformed into new benzene rings by nitroethylene. Some hurdles still remain, but the day is drawing nearer when SWNTs of predefined diameter and chirality will be “made to order” by chemical methods in the laboratory.

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Atmospheric Research at Boston College

Figure: Measured Cloud Condensation Activity (k) as a function of measured oxygen to carbon (O/C) ratio for a range of aerosol materials (listed in the inset).

Research in the Davidovits laboratory focuses on the climate effects of atmospheric aerosol particles. Such particles are produced by both biogenic and human activities and play key roles in the development of cloud cover. All cloud droplets contain an aerosol particle that acts as a center for the condensation of water vapor. Only hydrophilic aerosols can serve as cloud condensation nuclei (CCN). Thus aerosol particles affect cloud cover and cloud stability, thereby impacting radiation balance.

The composition of aerosol is highly complex and subject to continual change in the atmosphere. Inadequate representations of aerosol cloud formation (CCN) activity and the resulting effects on cloud albedo and cloud lifetime represent particularly large sources of uncertainty in current climate models. A central aim of Professor Davidovits’s research is to obtain a simple, reliable characterization of the CCN activity of organic aerosol.

A single parameter formulation of CCN activity has recently been introduced and designated as k. This CCN activity parameter incorporates the initial (dry) particle size and the critical supersaturation of water vapor required to form cloud droplets. However, k also depends on particle composition. Thus, in principle, if the particle composition and size are known, CCN activity can be extracted from the k value.

A critical challenge to the success of this approach has been finding a way to represent simply the complex composition of organic aerosols. In recent work from the Davidovits laboratory, it has been found, within experimental accuracy, that k correlates linearly (or nearly so) with the aerosol (O/C) ratio, possibly independent of particle composition, particle diameter, or method of oxidation for a wide range of atmospherically-relevant organic aerosol precursors (see figure). Such a simple general correlation as this represents a significant step toward the goal of more accurately climate modeling.

This work was performed at Boston College as a joint project involving researchers from Boston College, Aerodyne, and two research groups from Finland.


“Relationship between aerosol oxidation level and hygroscopic properties of laboratory generated secondary organic aerosol (SOA) particles.” Massoli, P., A.T. Lambe, A.T. Ahern, L. R. Williams, M. Ehn, J. Mikkilä, M. R. Canagaratna, W. H. Brune, T. B. Onasch, J. T. Jayne, T. Petäjä, M. Kulmala, A. Laaksonen, C. E. Kolb, P. Davidovits, and D. R. Worsnop.. Geophys. Res. Lett. 37, L 24801, doi:10.1029/2010GL045258, 2010.

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Homegrown Catalyst Provides a Solution to a Problem in Synthesis

Discovery and development of catalytic methods for enantioselective conjugate additions of easily accessible C-based nucleophiles to unsaturated carbonyls faciliates the synthesis of a large assortment of enantiomerically enriched biologically active molecules and are therefore of great significance. Nevertheless, notable shortcomings persist in this area, particularly in the context of reactions furnishing quaternary carbon stereogenic centers. One deficiency relates to the paucity of protocols for catalytic conjugate additions of vinyl groups. Recent research in the Hoveyda group, reported in a recent Communication in the Journal of the American Chemical Society and performed by graduate students Tricia L. May and Jennifer A. Dabrowski, puts forward an exciting new method for enantioselective conjugate addition of silyl-substituted vinylaluminum reagents to five- and six-membered b-substituted cyclic ketones. Reactions are catalyzed by a chiral bidentate N-heterocyclic carbene-copper catalysts, originally designed and developed in the Hoveyda laboratories and used for a wide range of other important transformations such as enantioselective olefin metathesis, allylic substitutions, hydroboration and diboration reactions. The new catalyst is derived from air stable and commercially available CuCl2•2H2O, affording the desired products in up to 95% yield and >98:2 enantiomeric ratio. The resulting enantiomerically enriched vinylsilanes can be protodesilylated, converted to the corresponding vinyl halides, or oxidized to b-acyl-substituted enones in high efficiency. The utility of the NHC–Cu-catalyzed reaction has been demonstrated through a concise enantioselective total synthesis of natural product riccardiphenol B.

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New Water Splitting Electrodes Developed

One key challenge in materials research is how to tailor certain aspects of the intrinsic properties of a material without adversely altering others. This problem has made it extremely difficult to progress at a meaningful pace in several critical fields, such as efficient energy storage and high-capacity energy storage. In a recent study, Assistant Professor Dunwei Wang, demonstrates that this challenge can be addressed, at least in part, by forming heteronanostructures. Differing from simple nanostructures, heteronanostructures consist of multiple parts, each of which may be selected or tailored independently. When combined, they contribute to offering a collection of property features that are not found in simple nanostructures, thus making high-efficiency solar water splitting possible. An important step to this goal is recently reported in Angew. Chem. Int. Ed. (doi: 10.1002/anie.201004801), in which the Wang group achieved WO3-based photoelectrodes that are stable in neutral solutions (pH 7) and exhibit high catalytic activities. The discovery was enabled by the introduction of two material components – a TiSi2 nanonets for effective charge transport and an oxygen evolving catalyst (provided by their collaborator, Assistant Professor Harvey Hou, at the University of Massachusetts at Dartmouth) for fast charge transfer. Building on this success, the Wang group is presently applying similar design concepts to construct electrodes that will permit practical water splitting as a form of energy harvesting and storage.

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The total synthesis of sclerophytin A by the Morken Group

The oxygenated cembrane diterpenes are a large class of natural products comprising cladiellins, briarellins, asbestinins, and sarcodictyins. Their intricate chemical structures have captivated many in the synthetic chemistry community. In addition to intriguing structures, these compounds also tend to possess potent biological activity. Amongst the cladiellins, sclerophytin A is a particularly striking compound; it was isolated from the marine soft coral Sclerophytum capitalis and found to exhibit remarkable potency against mouse leukemia cells (cytotoxic at 1 ng/ml versus L1210 cell line), yet its original structural formulation was incorrect. Total syntheses by the Paquette and Overman groups established the correct formulation of sclerophytin A to be that depicted in Scheme 1. As a means to examine the utility of the stereoselective Oshima-Utimoto reaction in chemical synthesis, Professor James Morken's research group has been attracted to the target sclerophytin A and its structural relatives. In this study, we show that (-)-sclerophytin A can be constructed in 13 steps from geranial. Highlights from the synthesis are a stereoselective Oshima-Utimoto reaction, a Shibata-Baba indium-promoted radical cyclization, and a novel stereoconvergent epoxide hydrolysis.

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This work was highlighted in Nature Chemistry. See: http://www.nature.com/nchem/reshigh/2010/1110/full/nchem.927.html


Professor Larry McLaughlin's research featured in Chemical & Engineering News

Janus wedge

To control the expression products from unwanted genes including those from oncogenes, transformed cells and viral infections, it would be very valuable to target and inhibit the process of RNA transcription. Gene-specific pharmaceuticals would result. This process begins with the ability to recognize specific double-stranded DNA sequences. The natural processes for such recognition and control relies typically with proteins (suppressors and repressors) binding to DNA and selectively preventing transcription. Our understanding of the recognition processes between proteins and specific DNA sequences is not sufficient to design and prepare a protein product to target a unique double-stranded sequence.

The use of a single strand of DNA to target duplexes is feasible, but to date the one residue to one base pair format (resulting in DNA triplexes) has been limited to polypurine sequences; targets that are not biologically very prevalent, particularly in the DNA sequences of interest. We are developing a fundamentally new type of recognition format in which a third strand of DNA (or the related peptide nucleic acid, PNA) inserts itself between two Watson-Crick faces of the base pairs of the target duplex. The resulting Janus-Wedge (J-W) triplex (Janus after the Roman god often depicted with two faces) can be generalized to any duplex target. The success of this project will lead to a new generation of gene-specific pharmaceuticals.

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The Gao Group Publishes a VIP in Angewante Chemie

Gao and Zheng image

The latest research of the Gao group, appearing as a VIP article in Angewante Chemie, has ushered a new modality of molecular recognition into protein design. VIP (Very Important Paper) designation in Angewante Chemie represents the highest recommendation that a manuscript may receive in peer review. According to the journal, less than 5% of their manuscripts receive this distinction.

Well-programmed assembly of biomolecules serves as the foundation of the complex and hierarchical organization of biology. The exquisite specificity of molecular assembly is driven by a collection of noncovalent forces, including the well-characterized hydrogen bonding and electrostatic interactions. The report by Professor Jianmin Gao and his student Hong Zheng demonstrates for the first time that the quadrupolar interaction between aromatic rings can be utilized to program the specific assembly of protein molecules.

Aromatic compounds – benzene, for example – display an electronegative center and electropositive edge. This uneven electron distribution gives zero dipole, yet a big quadrupole moment. Perfluorinated benzene exhibits an opposite quadrupole and tends to bind benzene in a face-to-face geometry. To evaluate the potential of this unique noncovalent interaction in protein design, Gao and Zheng replaced all four aromatic residues in the core of a homodimeric protein with the perfluorinated variants. Structured as a homodimer on its own, the fluorinated mutant disrupts the native dimeric fold of the wild type protein to form heterodimers exclusively. The homodimer to heterodimer conversion is attributed to the cross affinity of aromatic side chains and the perfluorinated analogues. The authors further quantified the energetic scale of the quadrupole interaction between phenyl and perfluorophenyl moieties to be ~1.0 kcal/mol, comparable to that of weak hydrogen bonds.

The report by Gao and Zheng clearly demonstrates the aromatic quadrupolar interaction can direct protein-protein interactions in water. This new recognition element greatly expands our toolbox for programming protein assemblies in order to achieve novel materials for biomedical applications.

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Professor Kian Tan's Research is Highlighted in Chemical & Engineering News

Tan scaffolding ligand

A bifunctional amine-phosphine ligand designed to simultaneously bind the substrate and catalyst in hydroformylation reactions is proving to be a versatile directing group for organic syntheses, report Professor Kian L. Tan and coworkers of Boston College (J. Am. Chem. Soc., DOI: 10.1021/ja1036226). Tan’s group has previously shown how this so-called scaffolding ligand’s amine group helps bind an olefin substrate and the phosphine group helps bind a rhodium catalyst, leading to regio- and stereoselective hydroformylation of mono- or disubstituted olefins. Stoichiometric amounts of phosphorus-based directing groups were previously required to carry out such reactions, but by reversibly binding the substrate and catalyst, only a catalytic amount of the scaffolding ligand is needed. Tan, Xixi Sun, and Kwame Frimpong have now demonstrated that the scaffolding ligand selectively directs formation of a quaternary carbon, rather than preferentially forming a lactone, in the hydroformylation of styrene-based substrates (reaction shown, R = aryl groups). A general method of using hydroformylation to create highly substituted carbon centers in olefins had been lacking, the researchers note. The synthesis “demonstrates the power of directing groups to overturn inherent selectivities of reactions,” they write.